Structural biology of bacterial secretion systems in gram-negative pathogens--potential for new drug targets

Gram-negative bacteria have evolved diverse secretion systems/machineries to translocate substrates across the cell envelope. These various machineries fulfil a wide variety of functions but are also essential for pathogenic bacteria to infect human or plant cells. Secretion systems, of which there are seven, utilize one of two secretion mechanisms: (i) the one-step mechanism, whereby substrates are translocated directly from the bacterial-cytoplasm to the extracellular medium or into the eukaryotic-target cell; (ii) the two-step mechanism, whereby substrates are first translocated across the bacterial-inner membrane; once in the periplasm, substrates are targeted to one of the secretion systems that mediate the transport across the outer membrane and the release outside the bacterial cell. This review describes in details the main structural features of these secretion systems. Structural biology offers the possibility to understand the molecular mechanisms at play in the various secretion systems. It also helps to design specifically drugs that can block these machineries and thus attenuate the virulence of pathogenic bacteria.

Survivin expression in breast cancer predicts clinical outcome and is associated with HER2, VEGF, urokinase plasminogen activator and PAI-1

BACKGROUND

Survivin, a novel inhibitor of apoptosis, is one of the most cancer-specific proteins identified to date. In this study we (a) evaluated the association between survivin and HER2, vascular endothelial growth factor (VEGF) and uPA/PAI-1 expression and (b) defined its effect on clinical outcome in a large breast cancer patient cohort.

PATIENTS AND METHODS

Survivin expression was measured by ELISA in primary breast cancer tissue extracts from 420 patients with long-term clinical follow-up.

RESULTS

Survivin was detected in 378 (90%) of the 420 primary breast cancer cases. Increased survivin levels were significantly associated with high nuclear grade (P < 0.0001), negative hormone receptor status (P = 0.0028), HER2 overexpression (P = 0.0094), VEGF expression (P < 0.0001), high uPA (P = 0.0002) and PAI-1 levels (P = 0.0002). Using the 25th percentile (1.4 ng/mg) as a cut-off point, patients expressing elevated survivin had a significantly worse disease-free survival (DFS: P = 0.0007, RR 1.97) and overall survival (OS: P = 0.0009, RR 2.11) compared with patients expressing lower levels of survivin. In multivariate analysis, this prognostic value of survivin was independent of both traditional and novel clinicopathologic factors for both DFS (P = 0.0076, RR 1.72) and OS (P = 0.0155, RR 1.76).

CONCLUSIONS

The independent prognostic relevance of survivin, when combined with previous data from model systems implicating survivin in the inhibition of apoptosis, suggests that survivin may be a suitable target for future therapeutic strategies.

Cloning and expression of L-asparaginase gene in Escherichia coli

The L-asparaginase (ASN) from Escherichia coli AS1.357 was cloned as a DNA fragment generated using polymerase chain reaction technology and primers derived from conserved regions of published ASN gene sequences. Recombinant plasmid pASN containing ASN gene and expression vector pBV220 was transformed in different E. coli host strains. The activity and expression level of ASN in the engineering strains could reach 228 IU/mL of culture fluid and about 50% of the total soluble cell protein respectively, more than 40-fold the enzyme activity of the wild strain. The recombinant plasmid in E. coli AS1.357 remained stable after 72 h of cultivation and 5 h of heat induction without selective pressure. The ASN gene of E. coli AS1.357 was sequenced and had high homology compared to the reported data.

[The upconversion "characteristic saturation phenomenon" of ErYb:ZBLAN glass excited by 966 nm diode laser]

This paper researches the upconversion luminescence of Er:ZBLAN and ErYb:ZBLAN glasses excited by 966 nm diode laser. It is found that there is a new kind of "characteristic saturation phenomenon". It is that the log-log plot's slope of upconversion luminescence intensity upon laser power of ErYb:ZBLAN glass is decreased clearly than that of Er:ZBLAN, and both of their log-log plots are rather good straight line. This upconversion mechanism is a new kind of "diffusion-transfer" mechanism, that is energy diffusion among Yb3+ ions sequential followed by energy transfer between Er(3+)-Yb3+ ions. The "characteristic saturation phenomenon" is just resulted from energy diffusion.

[Study on the structure of epimerides of cycloclausenamide]

AIM

To study the structures of the epimerides of cycloclausenamide.

METHODS

The structures of compound I, extracted from Clausena lansium (Lour.) Skeels, and synthesized compound III were determined by single crystal X-ray diffraction analysis. The stereo-structures of compound II and IV were also built up through Tripos force field based on crystal structures of compound I and III.

RESULTS

The molecular formula and molecular weight were found to be C18H17O2N and 279.34 respectively. Compound I crystallized in monoclinic system, space group P2(1) with a = 0.5928(1), b = 1.5014(1), c = 1.6190(1) nm, V = 1.4410(3) nm3, Z = 4, Dx = 1.288 g.cm-3, Rf = 0.075, Rw = 0.073(w = 1/sigma 2|F|), S = 3.983; compound III crystallized in triclinic system, space group P1 with a = 0.5667(1), b = 1.2934(1), c = 2.1119 (1) nm, alpha = 102.17(1), beta = 90.25(1), gamma = 102.65(2) degrees, V = 1.4770(5) nm3, Z = 4, Dx = 1.224 g.cm-3, Rf = 0.047, Rw = 0.051(w = 1/sigma 2|F|), S = 0.467.

CONCLUSION

These results showed that compound I and III both are cycloclausenamide except that the directions of the phenyl group on C6 are different. Cycloclausenamide can form 4 pairs of epimerides but the directions of the phenyl group does not affect their energy in free state.

[The effect of ischemic preconditioning on adenosine phosphate content after spinal cord ischemia in rabbits]

AIM

To study the effect of ischemic preconditioning (IP) on adenosine phosphate content after spinal cord ischemia in rabbits.

METHODS

We established a lumbar spinal cord ischemia model by inflating the balloon of a 4F Swan-Ganz catheter positioned in the abdominal aorta of the rabbits. All rabbits were divided into sham-operated group, ischemic group and IP group. Adenosine phosphate content in spinal cord of all rabbits was determined at 4 hours, 2 days and 5 days after spinal cord ischemia by using reverse phase HPLC.

RESULTS

ATP content in lumbar spinal cord of rabbits in ischemic group was markedly reduced (P < 0.01 vs sham-operated group). But in IP group, it was obviously elevated at each time point compared with ischemic group (P < 0.01).

CONCLUSION

IP can significantly elevated ATP content in postischemic lumbar spinal cord in rabbits. This may be one of the mechanisms that IP can protect spinal cord from ischemic injury.

[An effect enhancement mechanism of up-conversion luminescence--up-conversion sensitization]

The research of frequency up-conversion has been developed greatly in recent ten years. In order to achieve its applications, it needs to enhance the up-conversion efficiency further greatly, which is the core problem of up-conversion. Because of the specialty of Yb3+ ion energy level, Yb3+ can greatly enhance up-conversion luminescence of co-doped rare earth ion activator through energy transfer. Meanwhile it may not cause the obvious fluorescence quenching. Thus it is very significance to investigate up-conversion sensitization which Yb3+ ion acts as a sensitizer. It is more important that it is quite urgent to combine up-conversion efficiency and material property to develop up-conversion. This paper reviews the proposing and developing process of up-conversion sensitization. The achievement of up-conversion sensitization field especial the originate fruit in indirect up-conversion sensitization obtained by China are introduce emphatically.

Genomic structure and chromosomal localization of the gene encoding TRAX, a Translin-associated factor X

The TRAX gene encodes a Translin-associated 33-kDa protein partner, TRAX. The TRAX protein has extensive amino acid homology with Translin, and contains bipartite nuclear targeting sequences, suggesting a possible role in the selective nuclear transport of Translin lacking any nuclear targeting motifs. In the present study, genomic clones of the human TRAX gene were isolated to determine the complete genomic organization. The genomic structure of the human TRAX gene was similar to that of the human Translin gene, consisting of six exons and five introns, encompassing approximately 27kb in genomic DNA. Northern blot analysis revealed a predominant transcript of approximately 2.7kb, and its distribution in various tissues was like that of Translin. Chromosomal mapping by fluorescence in situ hybridization (FISH) analysis allowed localization of the TRAX gene to human chromosome lq41.

In vivo biological effects of various forms of thrombopoietin in a murine model of transient pancytopenia

Thrombopoietin (TPO) is the natural regulator of platelet production in the bone marrow of mammals. This cytokine also seems to play an important role in the development of the erythroid lineage when recovering from anemic conditions. Here we study the effects of various TPO molecules on the recovery of hematopoietic lineages in a mouse model of pancytopenia. Based on previous animal experimentation and clinical experience with other hematopoietic cytokines, we found that daily dosing with TPO augmented the recovery of both the megakaryocyte and erythroid lineages in a mouse model of pancytopenia. However, further experiments showed that no benefit was gained by using more than a single dose of recombinant murine (rm)TPO(335) given 24 h after the initiation of the myelosuppressive treatment. This response to a single dose of rmTPO(335) is dose-dependent. However, the response was attenuated when a truncated, short half-life TPO molecule (rmTPO[153]) was used. Increasing the half-life of the molecule with 10 kDa polyethylene glycol (PEG) does not improve the response. Only when larger PEG molecules (20 kDa or 40 kDa) are linked to the rmTPO(153) is the response to single doses restored to the level of the full-length molecule. These data suggest that, unlike our experience with other cytokines, the commitment of progenitors to a megakaryocytic cell line is accomplished by a single short exposure to TPO.

[Determination of Avermectin B1 content by first-order derivative spectrum analysis]

After acquiring the ultraviolet absorption spectrum and its first order derivative spectrum distributions of Avermectin B1 by using UV-2401 PC spectrophotometer and spectrum-measuring system, we found that coordinates of the peak A252.2 and the valley A256.8 are not disturbed by the different forms of preparations. We considered that delta A = A252.2-A256.8 is the reliable ground for quantitative analysis of Avermectin B1 content in order to reduce error. The standard regression equation c = 272.7 delta A-0.161 and the related coefficient r = 0.9996 are obtained through measurements of the standard preparations. Then the Avermectin B1 content and the average recovery rate of different preparations are determined and the result indicates that this method is especially suitable for determining the Avermectin B1 content.

Construction and structural modeling of a single-chain Fv-asparaginase fusion protein resistant to proteolysis

In this study, we construct a fusion protein composed of L-asparaginase (ASNase; from Escherichia coli AS 1.357) and a protective single-chain Fv (scFv), which was selected from a phage-display scFv library from our previous studies. The antibody moiety of the fusion protein was fused to the N-terminus of the enzyme moiety via a linker peptide, (Gly(4)Ser)(6). Recombinant plasmid pET-SLA was constructed to express scFv-ASNase fusion to high levels in E. coli and the expressed product was found to form inclusion bodies. We obtained a soluble fusion protein by refolding and purification. The soluble fusion protein exhibited about 82% of the enzymatic activity of the native ASNase at the same molar concentration, and had a K(m) value similar to that of the native enzyme for the substrate L-asparagine. Importantly, the fusion protein was more stable than native ASNase. In addition: (1) following treatment with trypsin, alpha-chymotrypsin, and rennet, at 37 degrees C for 30 min, scFv-ASNase fusion retained 94.0%, 88.8%, and 84.5% of its original activity, respectively, whereas native ASNase became inactive; and (2) ScFv-ASNase fusion had a much longer in vitro half-life (9 h) in serum than the native enzyme (2 h). The three-dimensional structure of the fusion protein was obtained by modeling with the Homology and Discover modules of the INSIGHT II software package. On the basis of the structural evidence and biochemical properties, we propose that the scFv moiety of the fusion protein may confer ASNase moiety resistance to proteolysis as a result of both steric hindrance and a change in the electrostatic surface of the enzyme.

Receptor-selective variants of human vascular endothelial growth factor. Generation and characterization

Vascular endothelial growth factor (VEGF) is a pleiotropic factor that exerts a multitude of biological effects through its interaction with two receptor tyrosine kinases, fms-like tyrosine kinase (Flt-1) or VEGF receptor 1 and kinase insert domain-containing receptor (KDR) or VEGF receptor 2. Whereas it is commonly accepted that KDR is responsible for the proliferative activities of VEGF, considerable controversy and uncertainty exist about the role of the individual receptors in eliciting many of the other effects. Based on a comprehensive mutational analysis of the receptor-binding site of VEGF, an Flt-1-selective variant was created containing four substitutions from the wild-type protein. This variant bound with wild-type affinity to Flt-1, was at least 470-fold reduced in binding to KDR, and had no activity in cell-based assays measuring autophosphorylation of KDR or proliferation of primary human vascular endothelial cells. Using a competitive phage display strategy, two KDR-selective variants were discovered with three and four changes from wild-type, respectively. Both variants had approximately wild-type affinity for KDR, were about 2000-fold reduced in binding to Flt-1, and showed activity comparable with the wild-type protein in KDR autophosphorylation and endothelial cell proliferation assays. These variants will serve as useful reagents in elucidating the roles of Flt-1 and KDR.

Characterization of l-asparaginase fused with a protective ScFv and the protection mechanism

A fusion protein of the protective scFv linked to the C-terminus of ASNase via (Gly(4)Ser)(6) peptide was constructed. The ASNase-scFv fusion protein expressed in Escherichia coli exists mainly in the form of inclusion bodies, and a small amount of it was soluble. The soluble form was purified by four-step purification and it has been demonstrated that ASNase-scFv fusion exists as a dimer. By assay of the stability against proteolysis, the ASNase-scFv fusion was found to be more stable than native ASNase but less stable than scFv-ASNase fusion. The results of immunological assay indicated that the immunogenicity of the fusion proteins increased while their binding capacity with the anti-ASNase serum decreased by comparison to the native ASNase. Moreover, here the comparison of the basic physical and chemical properties of the ASNase-scFv fusion, scFv-ASNase fusion, and native ASNase is presented. Based on the structural evidence and the biochemical analysis described in this paper, the protection mechanism proposed in our previous study was further supported. The scFv moiety of the fusion protein may confer the ASNase moiety resistance to proteolysis as a result of both steric hindrance such as blocking the cleavage sites of trypsin and a change in the electrostatic potential surface of the enzyme.